Microbial fuel cells can theoretically be useful in various applications, such as in wastewater treatment, and/or in renewable energy production, but serious issues of efficiency and reliability still have existed. While a microbial fuel cell according to the present invention may be used in place of conventional microbial fuel cells (MFCs), it can be uniquely designed to advantageously generate power in air-less and oxygen-less environments, such as outer space. Despite ongoing research in the area of microbial fuel cells, their previous output power density and efficiency of fuel conversion to energy performance has limited MFCs' production and application. There remains a continuous demand for improvement in the field of microbial and bio fuel cells.
Microbial fuel cells may be used as a method of wastewater treatment coupled to electricity generation, or as a method of renewable energy generation from non-waste products. Some bio-electrochemical systems may be used as a method of fuel (hydrogen or methane) generation. Microbial fuel cells can, in theory, use any microbial digest or metabolic product for fuel to generate electricity. Wastewater, biomass, or any other substrate of choice, can be provided as a biodegradable fuel oxidized by microorganisms in microbial fuel cells directly. Alternatively, the wastewater is biodegradable to produce other products that can be oxidized by microorganism in a microbial fuel cell.
In their most conventional form, MFCs utilize microorganisms to catalyze the oxidation reaction at the anode. The diversity of microorganisms that can be used as the catalyst at an anode widens the fuel choices in comparison with the current preprocessed fuel choices for fuel cells. This is due to existence of specific reaction pathways in the presence of microorganisms, which can reduce the anodic overpotential of fuel oxidation.
Microbial and biological fuel cells offer the potential to employ an organism or a part of the organism (such as enzymes or protein extracts from the organisms) to convert energy stored in organic carbon compounds (waste) into electricity. A biological fuel cell refers to an energy generation device wherein at least one of the two electrochemical reactions (i.e., oxidation at anode (negative electrode) and reduction at cathode (positive electrode)), is catalyzed using an organism or a part of the organism. Biofuel cells that employ microorganism are more specifically classified as MFC. The catalytic activity of MFC is generated, e.g., by microbes (generally, bacteria) that attach to the conductive surfaces of electrodes (anodes and/or cathode) and form electrochemically active biofilms.
In cases that MFCs contain anodophilic microorganisms in anodic compartment, microbes within a biofilm at the anode can enzymatically extract electrons from organic components in an aqueous solution and transfer the electrons to the electrode, while producing protons. The protons generated typically pass through a cation-permeable membrane (e.g., a Nafion membrane) towards cathode. The electrons flow from the anode through the electrical connection between the anode and cathode, producing a current. At the cathode, the electrons can combine protons and oxygen (typically, final electron-acceptor or oxidant) to form water. In the embodiments of MFCs with cathodophilic microorganisms, microbes within the biofilm at the cathode enzymatically transfer electrons from the cathode to an oxidant. The power produced by such a fuel cell can be used as a power supply.
Since MFC systems are designed to immediately move the electrical energy away from the anode to the cathode through electrical current generation, the microbes are unable to use the energy for growing and for building biomass. Furthermore, the movement of energy away from the microbes also accelerates microbial metabolism and increases primary waste reduction rates.
The electrocatalytic function of microorganisms for fuel oxidation can be understood by studying the metabolism of the fuel in living organisms. We find fuels can include, but are not limited to, sugars (for example, glucose, fructose, lactose, etc.), organic acids and metabolic intermediates (for example, lactate, acetate, etc.), biopolymers (for example, cellulose, chitosan, etc.), and mixed waste streams (for example, compost, wastewater, food wastes, etc.).
For a fuel such as acetate, the metabolism mechanism can be divided into two separate chemical half-reactions. The first half-reaction is the oxidation of acetate to produce carbon dioxide, protons, and electrons at the anode:CH3COO−+4H2O→2HCO32−+9H++8e−and the second half-reaction is the reduction of an electron acceptor such as oxygen to form water at the cathode:2O2+8H++8e−→4H2OWhen the oxidation half-reaction and reduction half-reaction are combined, the overall reaction for metabolism of acetate is:CH3COO−+2O2→2HCO32−+H+
The energy released in this reaction is used in living organisms to generate ATP, which serves as an energy carrier in cells to drive biochemical reactions. The energy is harnessed by coupling the generation of protons and electrons to enzymes that generate ATP. If these electrons and protons are diverted from ATP generation in the cell, they can be used to power a fuel cell. In theory, the oxygen reduction coupled with oxidation of acetate creates a maximum voltage of 1.1 V. However open circuit potentials of a well-controlled laboratory MFC usually do not exceed 0.8 V. While MFCs can be used to convert organic contaminants in wastewater into electric energy, the low voltage has been a problem limiting practical applications in large scale wastewater treatment plants.
To form a biocatalyst on the anode, the anodophilic bacteria can be incubated by placing the anode in the organic material oxidizable by the bacteria under oxidizing reaction conditions including maintenance of substantially anaerobic conditions. In the case of anodophilic microbes that transfer electrons through a mediator, the microbes are not required to be present on the anode surface, but may exist elsewhere in the anodic chamber, and still produce electrons that are successfully transferred to anode through the mediator.
There are a variety of technical problems with MFCs known in prior art. The traditional choice of oxidant for fuel cells, and microbial fuel cells is oxygen. This choice is mostly rationalized based on the fact that oxygen is naturally available in the air. However, the kinetics of oxygen reduction reaction is slow, and the overpotential of this reaction is about 400 mV. Even to achieve such potentials, expensive catalysts of platinum metal group are required. In old art MFCs with air or oxygen cathodes, a membrane as a barrier to oxygen entry into the anode chamber is placed between the anodic and cathodic chambers. Most commonly, a cation or proton exchange membrane, such as a perfluorinated sulfonic acid polymer or NAFION, which is substantially impermeable to oxygen is used in MFCs for this purpose. Although it is well known that oxygen reduction reaction is sluggish and rate determining step in the MFC, the least amount of attention has been focused on new solutions for the cathode problem.
Oxygen solubility in water is quite low and greatly hinders half-cell reactions dependent on oxygen. Oxygen solubility in water is influenced by water temperature, atmospheric pressure, and salinity. At 25° C. in the presence of 1 atmosphere of air, oxygen concentration in freshwater is about 6 mg/L, and this value decreases at higher temperatures and higher salinity levels. Due to low solubility of oxygen in water, direct air cathodes were developed. While direct air cathodes improve the amount of oxidant availability at the cathode, water leakage can become an expensive obstacle. The outer surface of a cathode can be covered by a cathode diffusion layer (CDL), which is preferably bonded to the cathode, to prevent water leakage through the cathode from the interior of the reaction chamber. However, the CDL needs to be oxygen permeable, allowing gaseous oxygen or air to freely diffuse from the outside of reaction chamber into the catalyst in the cathode. Further, it may be necessary to include a hydrogen permeable hydrophobic polymer material, such as polytetrafluoroethylene (PTFB), to the CDL. To minimize the water leakage, the thickness of this material is varied or multiple layers are required. These measures can greatly inhibit the efficiency of the cell in operation.
Where fuel oxidation by the microorganisms is done in anaerobic conditions, using the oxygen as the oxidant dictates complete separation of anodic and cathodic compartments, otherwise the parasite reactions such as aerobic oxidation of fuel, which does not generate free electrons to be transferred the anode to produce an electric current, will drastically reduce the cell voltage and current output. The requirement for a barrier, (e.g., an ion-exchange membrane), between the anode and the cathode causes structural restrictions for the MFC system design. The presence of membrane brings up other issues such as additional cost, and added internal resistance of system that also reduces the output voltage of the system. The extra barriers add to the cost of the system, increase electrical resistance, and reduce coulombic efficiency and power production. Further, the barriers are commonly not 100% efficient in exclusion of oxygen from anodic section due to crossover from cathodic compartment to anodic compartment.
In the industry, MFC treatment systems are only demonstrated at pilot scale, mainly because MFCs with two chambers and an electrolyte membrane are difficult to scale up in the structure. Using the membrane not only increases cost for configuring the system, but also has a problem in that the membrane may be contaminated during operation. This issue limits the application of MFCs for wastewater treatment, as the membrane easily gets fouled due to suspended solids and soluble contaminants found in wastewater. Adding to the problems, mechanical strength of the electrolyte membrane has to be high to endure scaled up pressures.
In view of the above, we see a need exists for a fuel cell/battery operated on organic matter and providing more practical voltage outputs. It would be desirable to have MFCs that are not reliant on oxygen and do not require membrane separation of the electrolyte. The present invention provides these and other features that will be apparent upon review of the following.